Smaller, lighter and thinner word processors and personal computers have been developed every year, and those having the so-called laptop type or notebook type size have become today's main trend. Since the liquid crystal display of the laptop type or the notebook type is usually not luminescent by itself, it has a back light on its back surface to improve visibility. The back light is required to be thin and to illuminate the display surface uniformly.
Usually, a planar lighting device is used as the back light, and many kinds of planar lighting devices are used. For example, as shown in FIG. 1, one of them has a light source 11 on the back side of a diffusing plate 10, wherein the light is emitted from the back side of the diffusing plate 10 and emanates uniformly from the surface of the diffusing plate 10 or from a regulator plate 12 attached to the surface of the diffusing plate 10, as described in Japanese Patent Provisional Publication No. 257188/90.
However, the most widely used planar lighting device is that of a so-called edge-light type. The edge-light type planar lighting device has a light source on a side of a light guiding plate, wherein the light emitted from the light source emanates uniformly from the surface of the light guiding plate or from a plate attached to the light guiding plate, and is widely used because it is thin and light, etc.
Recently, along with the efforts to make thinner displays and color displays, there is an increasing demand for improved luminance of a planar lighting device as the back light for displays. Since a user of the display looks at the display from the front side in most of the time, it is necessary to regulate the direction of the emanated light from the planar lighting device so that the light emanates efficiently in the forward direction to improve the luminance using the same light source. However, if the light emanates within a too narrow range, a slight change in the direction of the user's sight will make the display invisible. Therefore, it is required to diffuse the light across an appropriate range.
The details are described here using this planar lighting device of the edge-light type as an example. In general, if a light source is provided on the edge side of a thin light guiding plate, a light which enters the light guiding plate through the edge surface repeatedly experiences total reflections and almost no light leaves the light guiding plate because of the difference in the refractive index between the air and the light guiding plate. In the planar lighting device, an irregular reflection layer is provided on the back surface of the light guiding plate and a reflector plate is further provided on the back side of the light guiding plate, so that the light which reaches the back side of the light guiding plate is irregularly reflected by the irregular reflection layer, and then goes out of the light guiding plate directly or after being reflected by the reflector plate. The irregular reflection layer is often formed by the dot-printing using paint including beads of small diameters.
However, the light emitted from the light source on the edge surface of the light guiding plate usually goes out from the light guiding plate at a very small angle from the surface. That is, the light has a strong directionality, as shown in FIG. 2.
A more detailed explanation of the above problem is given here by referring to FIG. 2. A transparent light guiding plate 20 has an irregular reflection layer (not shown) on its back surface. A reflector plate 21 is provided on the back side of the light guiding plate 20, and a linear light source 22 is provided on the edge side of the light guiding plate 20. The light emitted from the light source 22 travels through the light guiding plate 20 and irregularly reflects at the irregular reflection layer provided on the back surface of the light guiding plate 20. Then, the light either directly goes out of the light guiding plate 20 or reflects at the reflector plate 21 and then goes out of the light guiding plate 28. The light from point A has a strong directionality making a very small angle with the surface of the light guiding plate 20, indicated by B in FIG. 2.
Since it is rare that the user looks at the screen of the display at such a small angle with the surface of the light guiding plate 28, it is necessary to change the angle of the emanating light. In order to improve the directionality of the light, a method in which a light diffusing plate 23 is provided on the front surface of the light guiding plate 20 has been proposed, as shown in FIG. 3. The light diffusing plate 23 is a transparent plastic sheet applied with a paint containing a white pigment on its surface, or a transparent plastic sheet provided with fine unevenness on its surface by the mat treatment, the crimp treatment, or such. By providing the light diffusing plate 23, the light in the direction perpendicular to the diffusing plate 23 increases, indicated by C in FIG. 3.
FIG. 4 shows a detail of the light crystal display, illustrating the basic structure of the planar lighting device of the edge-light type. The planar lighting device 30 mainly includes a light guiding plate 32 consisting of a transparent plate having a dot pattern 31 on the back surface, at least one linear light source 33 of a cathode ray tube (fluorescent lamp) provided on at least one side of the light guiding plate 32, a reflector plate 34 provided behind the light guiding plate 32, and a light diffusing sheet 35 consisting of a resin plate including a light diffusing material or a resin plate provided with crimps on its surface, and a liquid crystal display element 36 is further provided in front of the planar lighting device 30 (refer to Japanese Patent Provisional Publication No. 244490/89, U.S. Pat. No. 4,775,222 and U.S. Pat. No. 4,729,068). The dot pattern 31 is a light scattering printed dot pattern formed on the back surface of the light guiding plate 32 so that the incident light from the light source 33 on the side goes out uniformly from all parts of the screen of the display, and it may be called a pseudo light source. The light diffusing sheet 35 functions in such a way that the back light source, i.e. the dot pattern 31 as the pseudo light source, is not visible and the screen is seen as a uniformly luminescent plane, when using the liquid crystal display screen.
However, in the device using the light diffusing plate 35, much of the light is emitted in directions not needed for the user and the light in the forward direction is scarce, so that the luminance of the light in the foward direction from which the user sees is low.
Conventionally, in order to make the dot pattern 31 invisible and increase the light diffusion efficiency, the following measures have been taken: i) coating a light diffusing material onto a plastic sheet surface or introducing it into the inside of the sheet, and ii) creating a crimp-like unevenness or a regular unevenness on the plastic sheet.
In the former measure i), the light diffusing sheet 35 consists of a sheet formed by an extruder from a material made of a resin such as polyester, polycarbonate or polymethylmethacrylate, mixed with a light diffusing material such as fine-powder of calcium carbonate, titanium oxide, short glass fiber or silicone resin particles containing polysiloxane bonds (refer to Japanese Patent Provisional Publication No. 140343/78).
In the latter measure ii), the sheet having a crimp-like unevenness or a regular unevenness on its surface is provided on the front side of the light guiding plate 32 or on at least one surface of the light diffusing sheet 35 (refer to Japanese Patent Provisional Publication No. 257188/90). Such a sheet is called "a light adjusting sheet".
In one example of the latter measure ii), a light adjusting sheet 37 consisting of a transparent prism sheet with convex ridges and concave ridges each having a triangular cross section, as shown in FIG. 5, is provided on the front side of the light guiding plate 32 so that the opposite surface with no convex ridges and concave ridges of the light adjusting sheet 37 is in contact with the light guiding plate 32 in a liquid crystal display, as shown in FIG. 6. In such a planar lighting device of the liquid crystal display, the strong directional light is redirected in the direction of the user's sight by the slopes of the triangular cross section of the convex ridges and concave streaks, and the emitting direction of the light is confined to a predetermined range and converged in the direction of the user of a wordprocessor or other such device, i.e. the normal direction of the screen, and emanates from the front surface of the light adjusting sheet 37, thus obtaining a higher luminance in the screen.
However, in the former case i), of the planar lighting device with the light diffusing sheet, if the amount of the light diffusing material contained in the light diffusing sheet 35 is increased to improve the light diffusion, the amount of the emanated light is decreased because of the shielding characterisity of the light diffusing material, resulting in insufficient brightness in the screen, indicated by the dotted line D in FIG. 7.
On the other hand, in the latter case ii), of the planar lighting device with the light adjusting sheet, it utilizes the prism effect of the convex ridges and concave ridges on the surface of the light adjusting sheet, and is superior to the former device in terms of the amount of the emanated light. However, since the directionality of the light is too strong and the incident direction and the outgoing direction of the light are too strictly defined, so that the light emanating direction is deflected in the forward direction within a narrow angle range and the sight angle of the screen tends to be exceedingly narrow, as indicated by E in FIG. 8.
Further, when the light adjusting sheet consisting of a prism sheet of a serrate cross section is placed in such a way that its flat back surface is brought into direct contact with the light guiding plate, interference patterns of Newton ring appear due to slight gaps generated between the back surface of the light adjusting sheet and the light guiding plate. The closer the convex ridges and the concave ridges are provided to the final outgoing surface, the lattice-like partition lines of the liquid crystal display surface and the ridge lines and bottom lines of the prism sheet cause interference (the Moire phenomenon), depending on the pitches of the convex ridges and concave ridges.
Further, the inventor experimented with this planar lighting device and found that there is a problem in that the irregular reflection layer provided on the back surface of the light guiding plate is visible, although it can indeed alter the direction of the light efficiently so that the light emanates in the direction of the user's sight. When a display using the liquid crystal is placed on the planar lighting device with the visible irregular reflection layer, the screen of the display is very hard to see.
Therefore, a light diffusing sheet 35 is required in the planar lighting device when it uses the conventional light adjusting sheet.